Steam generator



July 2, 1940.

G. W. SAATHOFF STEM GENERATOR A Filed oet. 1e, 19:57

3 Sheets-Sheet 1 INVENTOR. George W Yad/7017 July 2,1940- G. w.sAA'rHI-F 2,206,265

STIAI GENERATOR :man on. 1e, 1931 s shuts-sheet 2 INVENT OR.

July 2 '1940- G. w. sAAT'HcFF 2,206,265

STEAK GENERATOR Filed Oct. 16. 1937 5 Sheets-Sheet 3 l Fig 9 IN VENTOR.

george W Jaa/zoff ATTRNEY.

, Patented July 2, 1940 UNITED STATES PATENT OFFICE 16 Claims.

The present invention relates to the construction and operation of steamgenerators, and more particularly to steam generators designed for theburning oi finely divided solid fuel in suspension and steam generatorsincorporating a steam superheater within the space occupied by theconvection heated portion of the steam generating surface. Y

.Finely divided `solid fuel, such as puiverized coal, has beenheretofore burned in suspension under furnace conditions resulting inthe deposition oi' the non-combustible residue or ash on the furnacebottom in either a dry or a molten condition, depending mainly upon therelation l of the ash fusion temperature' to the .mean furnacetemperature, and the temperature at the furnace bottom. In general, whenthe mean furnace temperature is lower than the ash fusion temperature,the ash will collect on the gd'furnace bottom in a dry condition, andwhen above the ash fusion temperature the ash will collect in a moltencondition. When the separated ash is in either a dry or moltencondition. it may be readily removed while the furnace is in operation.When the ash is in an intermediate state of high viscosity, its removalwhile maintaining continuity of operation is extremely diilicult. l lCoals having a wide range of ash fusion temperature 4 are now burned inpulverized fuel furnaces. Such furnaces are usually designed for aspecified range of operating load with a certain kind of ash removalwhen burning pul- `verized coal having an'ash fusion temperature 'withina certain range. In such furnaces, the furnace temperature willnecessarily vary throughout the normal range of operation due to changesin the quantity of fuel burned.l It is also important in the operationof a steam 40 generator unit of the character described to minimize thedeposition of ash in va molten or sticky condition on the convectionheated steam generating and superheating surface beyond the furnace, andto limit the variation in steam' superheat temperature toa small range.

The main object of my invention is the provision of a steam generatorincluding an improved furnace construction and method of operating the'same adapted for the'burning of nnely divided solid fuel ofsubstantially any ash fusion temperature over a wide range of operatingload. while obtaining the ashI separating from the burning fuel in acondition suitable for easy removal while the furnace is in opera'-tion. A further object il the provision of a' (ci. 12a-235.1)

steam generator of the character described with a furnace constructionand mode of operation permitting the separation and discharge ofthe ashin either a dry or molten condition as indicated to be preferable by theash characteristics of the .fuel in use. A further object is theprovision of a steam generator of the character described with aconstruction and arrangement of the convection heated steam generatingand superheating surface providing a high steam generating capacity, lowdraft loss. minimum slag deposits, and an inherent regulation ofsuperheat temperature.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart for this specication. For a better understanding of the invention,its operating advantages andspeciilc objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which I have illustrated and described embodiments of myinvention.

Of the drawings:

Fig. 1 is a partly diagrammatic sectional elevation of a steam generatorconstructed and adapted to operate in accordance with my invention;

Fig. 2 is a section taken on the line 2 2 of Fig. l'with a portion ofthe refractory omitted;

Figs. 3, 4, 5, 6 and 7 are sections taken on the lines 3 3, 4 4, 5 5, 56, and 1 1 of Fig. 1 respectively, and

Fig. 8 is a view similar to Fig. 1 illustrating a modified construction.

The steam generator units illustrated in the drawings comprise ingeneral a two-stage` pulverized coal burning furnace, a three drum steamboiler of the vertical bent tube type, and an air heater, all relativelyarranged as hereinafter described. In the construction shown in Figs.l-7. the furnace is defined by a front wall 4Il, opposite side walls Il,a rearwardly lstepped arch or roof I2, and arearwardly slopingtransversely corrugated floor '|3. All of these boundary surfaces arefluid cooled by heat absorbing fluid conduit elements connected intotending between the front wall header I6 and the front upper drum of theboiler; the side walls II have vertical water tubes I8 extending betweena bottom header l0 and upper headers 20 and 20 arranged at differentelevations; and the furnace bottom I3 has water tubes 2| extendingbetween the front wall header I5 and a 'a transverse header 23 below theArear end of the floor.

'Ihe furnace is dividedv by a transverse baille or partition structure24 into a primary or high temperature section and a rearwardly adjoiningsecondary or low temperature section communicating at their lower ends.The partition structure 24 extends downwardly from the rear part of thefurnace arch in substantially vertical alignment with the rear end ofthe floor I3. The partition extends through approximately the upper halfof the furnace forming the rear wall of the primary section and leavinga gas exit from the primary section to the secondary sec-1 tion belowthe lower end of the partition As shown in Figs 1v and 2, the partitionis formed by' a transverse row of water tubes 26 having their upper endsbent rearwardly and connected to the front upper boiler drum Theportions of the tubes 26 forming the partition are provided withmetallic s'tuds, as shown in Fig 2, projecting at their front sides andinto the intertube spaces. Refractory material 21 is applied to coverthe studs and closethe spaces between the tubes 26 The portions of thetubes 26 below the partition 24 are arranged to form spaced tube screensacross which the furnace gases from the primary section must pass beforecontacting with the main lconvection heating surface of the steamboiler. As shown in Figs. 1, 3 and 4,'transversely spaced pairs oftubes, designated'26, are continued verof the furnace.

tically downward to the floor I3 and connected to the header 23. Theremaining tubes, designated 26", are bent downwardly and rearwardly in astaggered formation, as shown in Fig. 4, with their lower ends connectedto the bottom drum 3I of the boiler. The space between the rear side ofthe partition 24 and screen tubes 26 and the main convection surface ofthe boiler forms the secondary or low temperature section Whi1e asindicated in Fig. 1, it is desirable to wholly or partly coverthe tubesalong the-boundaries ofthe primary furnace section with refractorymaterial to secure the desired rate of heat absorption in that section,the

tubes along the boundaries of the secondary furl nace section arepreferably left bare to obtain a higher rate of heat absorption. Theheat release rates and heat absorbing surface in the two furnacesections are proportioned'so that the normal mean furnace temperature inthe primary section willv be substantially higher than that in thesecondary section.

, AThe bottomof the secondary section is partly 4 defined by an inclinedrear wall 21 and an ash hopper 28. Transversely spaced groups ofvertically aligned tubes 33 extend across the upper end of the ashhopper 28 from the header 23 and thence upwardly along the wall 21 tothe lower boiler drum 3I. As shown in Fig. 6, the portions of the tubes33 directly above the hopper are studded and covered by refractory 341.The

spaces-@between -the tube groups 33 along the y f ront side of the wall21 are occupied by a row of tubes 35 in transverse alignment with theinnermost row of tubes 33 so that these tubes can be lcovered withmetallic blocks 36 tov form an indifferent elevations connected by asteeply inclined intermediate section, with the tubes I1' burners 40 isenclosed by an air box or compart ment 4I which supplies preheatedsecondary air around and between the various fuel streams. A similargroup of fuel burners 42 and air .boxes 43 are arranged to dischargethrough the rear arch section downwardly and somewhat forwardly into thefurnace diverging at an acute angle relative to the partition 24. Withthe described fuel burner arrangement, in operation the burning fuelstreams from the low level burners 40 will pass downwardly and slightlyrearwardly to the floor I3 and then horizontally along the floor to thegas exit formed vbelow the partition 24. With the low level burners 40in use the furnace temperature adjacent the oor I3 will be relativelyhigh and normally above the ash fusion temperature of the coal. Thedescribed fuel path would thus convert or maintain any ash or slagdeposited on the oor in a molten condition. 'I'he path of the burningfuel'streams from the high level burners 42 however is mainly vertical,bending horizontally at its lower end toward the gas exit across thetubes 26a. The fuel streams from the high level burners willhave asubstantially longer path of travel downwardly in the furnace than thestreams from the low level burners and vsweep at the most only a smallportion of the door. This fuel path in conjunction with the substantialamount of radiant heat absorbing surface throughout its length isconducive to a more uniform distribution of the heat released l theunit. In accordance with my preferred mode of operation, the furnace isnormally operated so as to secure the deposition of ash on the furnaceoor in a dry condition, but if, due to changes in the rate of heatrelease necessitated by variations in the load, the furnace conditionsbecome such that the ash deposited on the oor is or tends to becomesticky, rendering it vextremely difficult to be removed while thefurnace is in operation, then the method. of fuel introduction ischanged to insure the ash depositing on the furnace floor being molten,and thus in a condition to permit its flow over the rear end of thefloor to the ash hopper.

'Ihe fuel requirements of most pulverized coal burning steam plants aresupplied at all times by coal having substantially similar combustioncharacteristics. When the furnace construction described is in use insuch a plant. only the high level burners 42 'are used at low andmoderate operating loads and the low level burners 4l are idle, althoughsecondary air can be supplied from the air boxes 4I if desirable. Inview of the resulting path of travel of the burning fuel from theburners 42, the normal mean furnace temperature under these conditionswill be relatively low, and the ash separated willdeposit and can bereadily removed in a dry condition by raking or blowing the same olf thefurnace floor I3 between the tubes 26'* into the hopper 28. As theoperating load increases, a corresponding increase in the fuel supply tothe furnace must be made with a resulting rise in furnace temperature.When the coal has a relatively low ash fusion temperature, the separatedash tends to become sticky and fuse on any substantial increase in load.Accordingly, on such an increase in the boiler load, with a relativelylow ash fusion teinperature coal inv use, the high level burners 42 arewholly or partly shut down and the fuel wholly or partly delivered tothe low level burners 4II, the amount of coal still introduced throughthe high level burners depending primarily on the furnace volumerequired for optimum combustion conditions. With the described fuelstream path from'the burners 40, the furnace temperature in the oor zonewill be substantially in-.

creased and the ash deposited on the furnace floor can be easily meltedor maintained in .a molten condition. 'I'he molten, ash flows down theinclined oor il between the tubes 26* and.

tube groups 3l into the hopper 28 from which it can be sluiced in a wellknown manner. With the foregoing method of operation low ash fusion ltemperature coal can be burned under desirable conditions for completecombustion in the main furnace chamber and separated ash collected onthe furnace floor in a readily removable form over a wider range of loadthan is possible in other furnaces.

The furnace construction and burner arrangement described are alsoparticularly adapted for use in steam plants where the use of aplurality of coals having substantially different ash fusiontemperatures is desirable from an economic standpoint. In such plantswhen the normal mean furnace temperature at the desired rate ofheatrelease is below the ash fusion temperature of the high ash fusion coal,the high level burners 42 alone are used for the high ash fusion coaland the ash removed in a dry condition. When the contemplated normalmean temperature is adjacent or above the ash fusion temperature of thelow ash fusion coal, the low level burners 40 alone are used tointroduce low ash fusion coal and the ash lremoved in a moltencondition. At low loads. the furnace `temperature conditions willusually be such thatlow ash fusion coal may be introduced through thehigh level burners alone and the ash removed in a dry condition.

Under all conditions the furnace gases leave the main furnace chamberthrough the gas exit openings between the tubes "265 and turn upwardlyat the rear thereof. In view of the cooling effect of the tubes 26 andthe substantial amount of radiant heatabsorbing surface in the secondaryfurnace section, the temperature of the gases will be rapidly reducedvand any ash remaining in suspension, if fused or molten, will be cooledto a dry condition. Ash separating in the secondary section whetheroriginally dry or solidified therein by cooling will drop into thehopper 28 and be removed simultaneously with the dry ash or slag fromthe primary furnace section.

In view of the relatively high location of the high level burners, it ispossible to locate the boiler convection heating surface at one side ofthe furnace and within the height of the furnace, thereby minimizing thehead room required for the unit. The convection surface comprises afront bank of vertically disposed widely spaced bent tubes 45l)extending between the drums 30 and 3| and spaced rearwardly from thepartition 24, and a divided bank-of tubes 5I vertically arranged betweenthe drum Il and the rear boiler drum 32. The upper drums 30 and 32 areconnected by steam circulators and water circulators in the usualmanner, and the drum 32 connected to the inlet end of a steamsuperheater 52 formed by multiple looped flat coils arranged side-bysidethe full width of the boiler and having their opposite ends connected toan inlet header 5l and outlet header 54 respectively. The superheatertube coilsextend downwardly from the top of the setting and terminate asubstantial distance above the level of the drum 3l for a purposehereinafter set forth. The steam boiler is of the single pass type, nobailles being associated with the tube banks or superheater, with aresulting low draft loss through that portion of the unit.

The spaced arrangement of the tube bank 5l Y rear side of the partition.'Ihe furnace gases from the top of the vertical gas pass flowhorizontally over the tube bank 5| and then enter the upper end of atubular air heater 60 which is connected to the space at the rear of thetube bank 5| adjacent the upper end thereof. At low loads the coldergases tend to collect in the lower part of the boiler setting. Thisnatural stratification of the gases is particularly noticeable at lowloads when the effect of the draft inducing mechanism is relativelysmall. As the load lncreases the greater volume of gases flowing throughthe boiler and the increased effect of the draft inducing mechanismcauses a more uniform distribution of the gas flow across the boilertube banks. The gases pass down through the air heater tubes to a stackconnection. A forced draft fan 6I and conduits 62, 63, and 64 provide acirculation of air under pressure `through the air heater to thesecondary air boxes 4| and 43.

Under light load conditions `most of the furnace gases on leaving theprimary furnace chamber will pass vertically upward at the rear side ofthe partition 24 and then substantially horizontally across the tubebank 50, superheater 52' and tube bank 5| to the air heater 60, thestack effect of the vertical gas pass contributing substantially to thispath of gas flow. Asthe boiler load increases and consequently thevolume of furnace gases, a more uniform, distribution of the gas flowwill occur throughout the height of the horizontal pathacross theconvection surface. 'I'he boiler is proportioned so that at full loadall of the heating surface is equally active across the horizontalportion of the gas path from its top to its bottom, while at light loadsthe part near the top will be subjected to a greater gas flow than thepart near the bottom.

This variation in gas distribution from the top to the bottom of theconvection surface in the horizontal gas pass is advantageously utilizedby the described arrangement of the superheater tubes-52 to.limit therange of variations in superheat temperatures. 4 When superheatingsurface is swept by all of the heating gases at all loads,

the undesirability of this result has been responsible for the variousschemes that have been heretofore suggested for by-passing some of thefurnace gases at higher loads. 'I'he present superheater arrangementprovides an inherent control of superheat temperatures in that thesuperheating surface is restricted to the upper portion of thehorizontal gas pass. At very light loads the horizontal gas stream mayextend down from the top for only part of the length of the verticalsuperheater tubes, but with increase of load the Vgas stream will extendfarther down and so the active heating surface will vary with load.Proportions may be selected so that substantially all of thesuperheating surface will be in thev main path of the gases even atlight loads. The space between the lower end of the superheater and thebottom drum 3f will receive a larger 4fraction of the gases at highloads than at low loads, and correspondingly decrease the fraction ofthe gases contacting with the superheating surface. T he arrangementdescribed thus pirovides superheat control without any special flowcontrol devices.

In Fig. 8 is illustrated the construction 'shown in Figs. 1-7 asembodied in a central station installation, corresponding parts beingdesignated by, like reference numerals. The main differences reside inthe construction and arrangement of the partition tubes, the location ofthe front upper drum, a reversed arrangement of the superheaterconnections, and the use of a special superheater baille and baillesupporting tubes.

While in accordance with the provisions of the ystatutes I haveillustrated and described herein the best forms of the invention nowknown to me, those skilled in the art will understand that changes maybe made inthe form of the :apparatus disclosed without departing fromthe spirit of the invention covered byk my claims, and that certainfeatures of my invention may sometimes be used to advantage without acorresponding use of other features.

I claim:

1. A furnace for burning finely divided solid fuel comprising wallsdefining the sides of a furnace chamber, an arch having burner ports atsubstantially dierent elevations therein, a closed oor forming thebottom of said furnace chamber, and fuel burners arranged to dischargestreams of finely divided solid fuel through said burner portsdownwardly towards said floor through ame paths of substantiallydifferent lengths relative to said fioor.

2. A furnace' for burning finely divided solid fuel comprising Wallsdefining the sides of a furnace chamber having a gas exit in the lowerpart of one of the side walls, an arch having burner ports atsubstantially different elevations therein, a floor forming the bottomof said chamber, and fuel burners arranged to discharge streams offinely divided solid,l fuel through said burner ports downwardly towardssaid floor through fiame lpaths of substantially different lengthsrelative to said gas exit.

3. A furnace for burning finely divided solid fuel comprising verticallydisposed front,`side and rear walls defining the sides of a furnacechamber having a gas exit in said rear wall, a closed oor forming thebottom of said chamber, a

stepped arch forming the roof of said furnace chamberand having burnerports therein at substantially different elevations, the burner port atthe higher elevation being nearer to the rear wall of the furnacechamber, and separate fuel burners arranged to discharge streams offinely divided solid lfuel through said burner ports toward said fioorthrough fiame paths of substantially different lengths relative to saidoor.

4. A furnace for burning flnely divided solid fuel comprising verticallydisposed front, side and rear walls dening the sides of a furnacechamber having a gas exit in said rear wall, a closed uid cooledinclined floor sloping towards its rear end, an ash hopper at the rearend of said floor adjacent said gas exit, a stepped arch forming theroof of said furnace chamber and having burner ports therein atsubstantially different elevations, the burner port at the higherelevation being nearer to the rear wall of the furnace chamber, andseparate fuel burners arranged to discharge streams of finelydividedsolid fuel through said burner ports toward said floor through flamepaths of substantially different lengths relative to said floor. l

5. The method of 'operating a furnace having fluid-cooled walls andpulverized fuel burnes'arranged at substantially different lengths offuel travel in the furnace relativeto the furnacefloor which comprisesat low loads introducing pulverized fuel through the burners having thelonger length of fuel travel in the furnace chamber and burning the fuelin suspension while passing through a flame path towards' the furnacefloor and at a heat release rate sumcient to maintain a furnacetemperature adjacent the floor below the ash fusion temperature andcause ash separating from the fuel stream to collect on the floor in adry condition, and at high loads introducing fuel through the burnershaving the shorter length of fuel travel in the furnace cham- Aber andburning the fuel in suspension while passing through a ame path sweepingacross the furnace oor and of substantially less length than the namepath at low loads and' at a heat release rate sufficient to maintain afurnace temperature adjacent the floor above the ash fusion temperatureand cause separating ash to collect on the'fioor in a molten condition.

6. 'I'he method of operating a furnace having fluid-cooledV walls andpulverized fuel burners arranged to downwardly' discharge atsubstantially different levels in the furnace which comprises at lowloads introducing pulverized fuel having a relatively low ash fusiontemperature through the high. level burners and burning the fuel insuspension while passing through a flame path u towards the furnacefioor and at a heat release rate suflicient to maintain a furnacetemperature adjacent the floor below 'the ash fusion temperature andcause ash separating from the fuel stream to collect on the door in adryvcondition, and at higherloads introducing fuel through the low levelburners and burning the fuel in suspension while passing through a flamepath sweeping across the furnace floor and of substantially less lengththan the fiame path at low loads and at a heat release rate sufficientto maintain a furnace temperature' adjacent the floor above the ashfusion temperature and cause separatingash to collect on th'e floor in amolten condition.

7. The method of burning a finely dividedsolid fuel having a relativelylow ash fusion temperature in a fluid-cooled furnace chamber over a wideoperating range while causing separated ash to collect in a readilyremovable condition on the furnace floor which lcomprises burning thefuel in suspension at high loads while'passing through 75 a flame pathsweeping across the furnace floor and at a heat release rate sumcient tomaintain a furnace temperature adjacent the floor above the ash fusiontemperature to cause ash separating from the fuel stream to collect onthe oor in a molten condition, and burning the fuel in suspension at lowloads while passing through a flame path of substantially greater lengthrelative to the floor than the ame path at high loads and at a heatrelease rate sufficient to maintain a furnace temperature adjacent thefloor below the ash fusion temperature to cause ash separat- ,ing fromthe fuel stream to collect onvthe floor in a drv condition.

8.' The method of operating a furnace having fluid-cooled walls andpulverized fuel burners arranged at substantially different lengths offuel travel in the furnace relative to the furnace floor which comprisesat one time introducing pulverized fuel having a relatively low ashfusion temperature through the burners having the shorter length oftravel and burning the fuel in suspension while passing through a namepath sweeping across the furnace oor and at a heat release ratesuilicient to maintain a furnace temperature adjacent the oor above theash fusion,

temperature and cause ash separating from the fuel stream to collect onthe floor in a molten condition, and at another time introducingpulverized fuel having a higher ash fusion temperature through theburners having the longer length of -fuel travel in the furnace chamberand at a heat release rate suilicient to maintain a furnace temperatureadjacent the floor below the ash fusion temperature and cause ashseparating from the fuel stream to collect on the floor in a drycondition.

9. 'I'he method of operating a furnace having pulverized fuel burnersarranged to downwardly discharge at substantially, different levels inthe furnace which comprises at one time introducing pulverized fuelhaving a relatively low ash fusion temperature through the low levelburners and burning the fuel in suspension while passing through a flamepath sweeping across the furnace floor and at a heat release ratesuilicient to maintain a furnace temperature adjacent the floor abovethe ash fusion temperature and cause ash separating from the fuel streamto collect on the floor in a molten condition, and at another timeintroducing pulverized fuel `having a higher ash fusion temperaturethrough the high level burners and burning the fuel in suspension whilepassing through a flame path of substantially greater length than theflame path of the lower ash fusion temperature fuel and at a heatrelease rate sufficient to maintain a furnace temperature adjacent thefloor below the ash fusion temperature and cause ash separating from thefuel stream to collect on the door in a dry condition.

10. A steam generator comprising a furnace chamber, a partition arrangedto divide said furnace chamber into primary and secondary sectionsserially connected below said partition, convection heated steamgenerating and superheating surface adjacent one side of said partitionreceiving furnace gases from said secondary furnace section andincluding a plurality of vertically disposed banks of water tubes havinga gas flow mainly transversely of the tubes thereof, and a steamsuperheater positioned lbetween said tube banks and having its lower endterminating above the lower end of said tube banks to provide a gasby-pass from said secondary furnace section around the lower end of saidsuperheater.`

, v. il

11. A steam generator comprising a furnace chamber, a partition arrangedto divide said furnace chamber into front and rear sections seriallyconnected below said partition, convection heated` steam generating andsuperheating surface adjacent the rear side of said partition receivingfurnace gases from said rear furnace section including a plurality oftransverse upper-drums, a transverselower drum, vertically disposedbanks of water tubes connecting said upper drums to said lower drumhaving a gas flow mainly transversely of the tubes thereof, and a steamsuperheater positioned between said water tube banks and having itslower end spaced fromv said lower drum to provide a gas by-pass fromsaid rear section between the lower end of said superheater and saidlower drum.

12. A steam generator comprising a furnace chamber, a partition arrangedto divide said furnace chamber into front and rear sections seriallyconnected below said partition, a closed oor forming thev bottom of saidfront furnace section, an ash hopper at the rear end ofY said floorforming ther bottom of said rear furnace section, tubes'extendingdownwardly along said partition and having their lowerend portionsrelatively arranged to form a plurality of relatively angularly arrangedtube screens, convection heated steam generating and superheatingsurface at the rear side of said partition and above 4said rear furnacesection including a plurality of transverse upper drums, a transverselower drum, vertically disposed banks of water tubes connecting saidupper drums to said lower drum, and a steam superheater positionedbetween said water tube banks.

13. A steam generator comprising a furnace chamber, a partition arrangedto divide said furnace chamber into front and rear sections seriallyconnected below said partition, a floor forming the bottom of said frontfurnace section, an ash hopper at the rear end of said floor forming thebottom of said rear furnace section, convection heated steam generatingand superheating surface adjacent the rearside of said partition andabove said rear furnace section including a plurality of transverseupper drums, a transverse lower drum, vertically disposed banks of watertubes connecting said upper drums to said lower drum and having agas'flow` mainly transversely of the tubes thereof, and a steamsuperheater positioned between said water tube banks.

14. A steam generator comprising a furnace chamber, a partition arrangedto divide said furnace chamber into front and rear sections seriallyconnected below said partition, a floor for-ming the bottom of saidfront furnace section, an ash hopper at the rear end of said doorforming the bottom of said rear furnace section, tubes extendingdownwardly along said partition and having their lower end portionsrelatively arranged to form a plurality of relatively angularly arrangedtube screens, convection heated steam generating and superheatingysurface at and spaced from the rear side of said partition and aboveand at the` side of said rear furnace section including a plurality oftransverse upper drums, a,

' transverse lower drum, vertically disposed banks of water tubesconnecting said upper drums to said lower drum, and a steam superheaterpositioned between said water. tube banks and having its lower endspaced from said lower drum to provide a gas by-pass from said rearfurnace section between the lower end of said superheater and said lowerdrum.

A 15. A steam generator comprising a furnace chamber, a partitionarranged to divide said furnace chamber into front and rear sectionsserially connected below said partition, a closed floor forming thebottom ofsaid front furnace section, fuel burners arranged atsubstantially diiferent elevations relative to said floor arranged todischarge streams of finely divided solid :fuel towards said fioor, an`ash hopper at the rear end of said iloor forming the bottom of said rearfurnace section, tubes extending downwardly along said partition andhaving their lower end portions relatively arranged to form a pluralityof angular-1y arranged tube screens, convection heated steam generatingand superheating surface at and spaced from the rear side of saidpartition and above and at the side of said rear furnace sectionincluding a plurality oi' transverse upper drums, a transverse lowerdrum, vertically disposed banks of water tubes connecting said upperdrums to said lower drum, and a steain superheater positioned betweensaid water. tube banks and having its lower end spaced from said lowerdrum to provide a gas by-pass from said rear furnace section between thelower end of said superheater and said lower drum.

16. 'I'he method of burning two pulverized coals of substantiallydifferent ash fusion temperatures in the same uid cooled furnace whichcomprises at one time introducing the pulverized coal having the -higherash fusion temperature into the furnace at a point providing arelatively long length of fuel travel in the furnace towards the furnacebottom and a heat release rate maintaining a furnace temperatureadjacent the furnace bottom below the ash fusion temperature, wherebyash separating from the fuel stream will deposit on the furnace bottomin a dry condition, and at another time introducing the pulverized coalhaving the lower ash fusion temperature into the furnace at a pointproviding a substantially shorter length of fuel travel in the furnacetowards the furnace bottom and a heat release rate maintaining a furnacetemperature adjacent the furnace bottom above the ash fusiontemperature, whereby ash separating from the fuel stream will deposit onthe furnace bottom in a molten condition.

GEORGE W. SAATHOFF. 25

